Porosity of Particles

Individual primary particles could be dense, while agglomerates are most likely porous. Therefore, it is desirable to quantitatively characterize the porosity and pore size and distribution of the agglomerates. For accessible pores, i.e., those that are not completely isolated from the external surface, they can be characterized by using two methods (i) gas adsorption, also known as capillary condensation and (ii) mercury intrusion porosimetry, also called mercury porosimetry for simplicity. The pore size can be diameter, radius, or width. Three types of pores have been classified according to their sizes micropores ( 2 nm), mesopores (2-50 nm), and macropores ( 50 nm). Generally, gas condensation is applicable to the measurement in mesopores, whereas mercury porosimetry is more suitable to macropores. 

At lower gas pressures, e.g., p/po 0.3, the adsorbed gases cover the surface of the solid by forming multilayers, whereas at higher pressures, the gas could condense to liquid due to the capillary force of a porous solid, which can be used to measure the pore size and the size distribution. When the adsorbed gases are condensed, the isotherm will have a hysteresis loop for a cycle of an adsorption and a desorption, represented by the Type IV and V isotherms, as shown in Fig. 4.3. 

If the capillaries are assumed to be cylindrical with a radius r, the relative gas pressiure in the capillaries for condensation can be described by the Kelvin equation  

When using the Kelvin equation to determine the pore size distribution, with the capillary condensation part of the Type IV isotherms, the adsorbing gas should be nitrogen, so that both surface area and pore size distribution can be obtained from one isotherm. If the volume of the gas adsorbed on external surface of the solid is smaller than that adsorbed inside the pores, the voliune of the pores can be obtained, when the volume of the gas adsorbed Vg is converted to a liquid or condensed volume 14- The relationship between Vg and is given by  

Because typical values of p/p are in the range 0.5-0.95, the gas adsorption is corresponding to the pore radius with values in the range of 1-20 nm. 

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The type of chosen polymer and additives most strongly influences the rheological and processing properties of plastisols. Plastisols are normally prepared from emulsion and suspension PVC which differ by their molecular masses (by the Fickentcher constant), dimensions and porosity of particles. Dimensions and shape of particles are important not only due to the well-known properties of dispersed systems (given by the formulas of Einstein, Mooney, Kronecker, etc.), but also due to the fact that these factors (in view of the small viscosity of plasticizer as a composite matrix ) influence strongly the sedimental stability of the system. The joint solution of the equations of sedimentation (precipitation) of particles by the action of gravity and of thermal motion according to Einstein and Smoluchowski leads 37,39) to the expression for the radius of the particles, r, which can not be precipitated in the dispersed system of an ideal plastisol. This expression has the form ... 

The porosity of particles suitable for packing HPLC columns depends on the size of molecules to be separated. Totally porous particles with a pore size of 7-12 nm and specific surface area of 150-400 m"/g are suitable for the separation of small molecules, but wide-pore particles with a pore size of 15-100 nm and relatively low specific surface area (10-150 nr/g) are required for the separation of macromolecules to allow easy access to the interactive surface within the pores. Packings with perfusion particles contain very broad pores (400-800 nm) throughout the whole particle interconnected by smaller pores. The mobile phase flows through the pores in the particle, which minimises both band broadening and column backpressure (111. Perfusion materials have been designed especially for the separation and isolation of biopolymers. 

Specific surface area was obtained using the B E T. method (Sbet)[1] In this method no assumption is made on the particles shape and the whole surface developed by the powder (including internal porosity of particles) is measured except for sealed pores. 

The size of the colloidal non-porous particles determines the specific surface area of an adsorbent (Eq. 3.12) and the porosity of particles is controlled by the average contact number of non-porous primary particles. 

Porosity. Part of the porosity of particles of commercial quicklime arises from the porosity of the limestone, and part from the decomposition process. The porosity of conunercially produced quicklime can be as high as 55% (by volume), when a porous limestone is lightly burned. Exposure to elevated temperatures results in sintering (see sections 15.4 and 15.5), which can reduce the porosity to below 25 %. Dead-burned dolomite has a porosity of about 10 %. 

The porosity of particles C and D was compared with that of the pre-formed Ludox TMA silica particles (Table 2). A slight increase in the surface area of the hybrid particles C and a high surface area for the silica particles D were observed. Porosity of 35 to 50 % was generated compared to the dense Ludox TMA silica particles this was also slightly higher than that for particles A and B obtained by the first route. The pore size distribution calculated by the BJH method also revealed the formation of mainly mesoporous particles C and microporous particles D witii pore sizes mostly below 50 A, conversely to particles C. 

The bulk density of a powder, which often is an important product quality aspect, depends, among others, on the inter-particulate porosity. The porosity of particle... 

A.B. Yu and N. Standish, Estimation of the porosity of particle mixtures by a linear-mixture packing model, Ind. Eng. Chem. Res., in print. 

The type of plasticizer and its interaction with PVC is important in selecting the polymer/plasticizer ratio (28). The PVC properties which affect this interaction are size distribution structure, shape, porosity of particles molecular weight and distribution of PVC and other chemicals used in the compounding recipe. Plasticizers are usually added in amounts greater than 20 parts per hundr parts of resin (phr). The reverse effect, unplasticization, is observed at lower concentrations. For most resin-plasticizer combinations at room temperature, a plasticizer threshold concentration must be passed before the normal plasticizer has an effects on physical properties. The extent to which Tg is depressed depends on the amount of plasticizer present. The usual concentration range for plasticizers is 20-85 phr. 

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